In a conventional telephone system, the majority of subscribers are connected to local exchanges via twisted conductor pairs, generally referred to as subscriber loops. Between the subscribers and the exchange, the subscriber loops are carried in cables each containing a large number of conductor pairs. The cables from the exchange feed smaller street cables from which the individual subscriber loops are ‘dropped’ to provide the final link to the subscriber. These twisted pair subscriber loops, which were originally installed to carry voice services, are now being used by the system operators to carry new services such as ADSL (asymmetric digital subscriber line) in the frequency spectrum above the base band frequencies used for the voice or POTS traffic. These services are added to the subscriber line at the exchange and may carry e.g. video and/or Internet traffic to the subscriber.
The currently employed ADSL service is a broad band technology which occupies a frequency band above that of voice to provide high bit rate (asymmetric) services to customers. Typically this service uses discrete multi-tone technology (DMT) at frequencies of about 20 to 100 kHz up to about 1.1 MHz at a downstream bit rate of 2 Mb/s and at an upstream bit rate of 200 kb/s. The ADSL service is usually delivered via a carrier based system using discrete multi-tone (DMT) in which the traffic is carried on a number of frequencies over the twisted pair to the subscriber.
There are now proposals to introduce a new higher bit rate interactive or symmetric service generally referred to as VDSL (very high bit rate digital subscriber line). This service is intended primarily for business customers to carry large volumes of data traffic to and from such customers. For most users the service will be provided over an existing twisted pair subscriber loop or link coupling that user's terminal to a local exchange so as to avoid the cost of installing e.g. a coaxial link to the subscriber.
A particular problem with the introduction of VDSL systems is that of managing and controlling the subscriber link to allow remote status and performance monitoring of the link and to perform management operations which require downloading of information and commands. A further requirement is that of providing synchronisation between the transmitters and receivers at the exchange and subscriber ends of the links. This synchronisation of transmitter and receiver is essential where the digital data transmitted over the link has been scrambled to provide an even distribution of binary ones and zeros.
In a conventional e.g. ADSL system, an engineering operations channel is provided by the allocation of bits, often byte oriented, in each superframe structure of a transmission line-code. This provides the transport of a set of so-called indicator bits and control messages.
Currently employed EOC arrangements suffer from the disadvantage that they are protocol specific and thus lack flexibility. In an attempt to reduce this problem, there have been proposals for inserting EOC information into an ATM channel. This has the advantage of common cell processing, but the need to accommodate fixed length cells has limited the efficiency of the technique.